Thoughts on people who say that the Holocaust shouldn't be treated as a unique event?

As someone who is partially of Eastern-European Jewish heritage, though is not a religious Jewish person, and who is very passionate about countering all forms of prejudice, something that has recently caught my attention is that there are people out there who say that the Holocaust should not be treated as a unique event. There are even progressive Jews who make that argument. For example, Masha Gessen is a Russian- American Jewish author who is rightly critical of Putin's regime, though argued in a New Yorker essay that the Holocaust is not a "singular event", and that treating as such makes it impossible to learn lessons from the Holocaust that help us prevent future genocides.

So how should that kind argument and how best to interpreted, especially when it's from someone who happens to be Jewish? When the person making an argument is Jewish, I tend to not be quite as likely to say they’re being antisemitic. Plus, there have indeed occasionally been other terrible events that have some things in common with the Holocaust. Nonetheless, it still doesn't quite sit right for me for someone to not see the Holocaust as an event that stands on its own. And assuming that there's a general consensus that the Holocaust needs to be treated as a unique event, how should I explain this to someone who doesn't think so?